Monnow Bridge (Welsh: Pont Trefynwy Welsh pronunciation: [pɔnt tre:vənʊɨ]), in Monmouth, Wales, is the only remaining fortified river bridge in Great Britain with its gate tower standing on the bridge. Such bridge towers were common across Europe from medieval times, but many were destroyed due to urban expansion, diminishing defensive requirements and the increasing demands of traffic and trade. The historical and architectural importance of the bridge and its rarity are reflected in its status as a scheduled monument and a Grade I listed building. The bridge crosses the River Monnow (Afon Mynwy) 500 metres (1,600 ft) above its confluence with the River Wye.
Monmouth had been a significant border settlement since the Roman occupation of Britain, when it was the site of the fort of Blestium. The River Wye may have been bridged at this time but the Monnow, being easily fordable, appears not to have had a crossing until after the Norman Conquest. According to the local tradition, construction of Monnow Bridge began in 1272 to replace a 12th-century Norman timber bridge. Through the medieval era, the English Civil War, and the Chartist uprising, the bridge played a significant, if ineffectual, role in defending Monmouth. It also served as a gaol, a munitions store, a lodge, an advertising hoarding, a focus for celebrations and, most significantly, as a toll gate. Much of the medieval development of Monmouth was funded by the taxes and tolls the borough was entitled to raise through royal charter. The tolls were collected through control of the points of entry to the town, including the gatehouse on Monnow Bridge.